1. Field of the Invention
This invention relates generally to radiation therapy equipment for the treatment of tumors, or the like, and specifically to a radiation therapy planning method for calculating the necessary beam strengths, in a therapy system for delivering multiple beams at different angles within a plane, so as to precisely regulate the dose of radiation within irregularly shaped zones within the patient.
2. Background Art
Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiating of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of "gantry" angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam are changed, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, some of which use multiple sliding shutters which, piecewise, may generate a radio-opaque mask of arbitrary outline.
As part of collimating the beam to the outline of the tumor, the offset angle of the radiation beam, with respect to a radius line between the radiation source and the center of rotation of the radiation source, may be adjusted to allow the treated area to be other than at the center of rotation. Simultaneously changing the offset angle and the width of the radiation beam as a function of gantry angle allows tumorous tissue having an irregular cross-section within a plane parallel to the radiation beam to be accurately targeted. The width and offset angle of the radiation beam may be controlled by the use of a multiple-leaf collimator.
Adjustment of the offset angle, center, and size of the radiation beam at various gantry angles allows considerable latitude in controlling the dose. Nevertheless, these approaches still impart a considerable amount of undesired dose to healthy tissue, especially where the tumor is concave or highly irregular.
A radiotherapy machine providing much reduced irradiation of healthy tissue is described in co-pending U.S. patent application Ser. No. 07/865,521, filed Mar. 19, 1992 and assigned to the same assignee as the present application. This radiotherapy machine has a number of radiation attenuating leaves in a rack positioned within the radiation beam before the beam enters the patient. The leaves slide into the radiation beam, in a closed state, to block a given ray of the beam, and out of the radiation beam, in an open state, to allow unobstructed passage of a given ray of the beam. By controlling the ratio of time spent in the open and closed states, each ray may be attenuated over a continuous range of intensities.
This ability to control not just the outline of the radiation but the intensity of each individual ray allows extremely precise control of the irradiation volume.
In theory, with the proper modulation of each ray of the beam as the beam revolves about the patient through a range of angles, the radiotherapy machine can precisely place the dose within even concave or highly irregular zones. Importantly, the radiation also can be accurately excluded from zones that include radiation sensitive organs or the like.
Such dose placement techniques require allocating the dose received by any volume element of the patient among the many possible rays of radiation at different gantry angles. This allocation, manifest as a set of beam intensities for each ray of the beam and for each gantry angle of the beam, is termed a treatment sinogram.
The above referenced co-pending application describes a method of producing a treatment sinogram using techniques analogous to the image reconstruction techniques of computed tomography ("CT") with the important distinction that CT image reconstruction derives an image of the patient by measuring the attenuations of a plurality of rays through the patient, whereas the radiotherapy system impresses a "dose image" on the patient by pre-attenuating a set of radiation beams at the various angles about the patient.
The sinogram generating techniques referred to above challenge the computing capability of present computer hardware.